1. Field of the Invention
This invention pertains to manufacturing selected components for reciprocating piston mechanisms, and more particularly to apparatus and methods for cracking engine and compressor connecting rods.
2. Description of the Prior Art
It is known to intentionally crack automotive engine connecting rods into a main rod section and a cap. Cracking occurs along a plane passing through the center of the crankshaft bore. The cap is later reassembled to the main rod section with a crankshaft throw captured therebetween. U.S. Pat. Nos. 3,751,080; 3,818,577; and 3,994,054 illustrate early developments of connecting rod cracking.
More recent examples of techniques and apparatus for performing the cracking process may be seen in U.S. Pat. Nos. 4,569,109; 4,684,267; 4,860,419; and 4,936,163. The prior art cracking apparatus often includes a pair of semi circular dies inserted into the crankshaft bore of a connecting rod. A force is applied to the dies to press them against opposite sides of the crankshaft bore. The dies stress the webs of material on the sides of the bore between the main rod section and the cap until the web separates. In most of the apparatus of the aforementioned patents, an input force is applied in a direction parallel to the plane of cracking. The input force is converted to a direction perpendicular to the plane of cracking by a wedge arrangement. Wedges possess the disadvantage of introducing friction into the cracking apparatus. Consequently, the input forces must be greater than are necessary merely to crack the connecting rods.
U.S. Pat. No. 4,754,906 shows a connecting rod manufacturing system in which a hydraulic cylinder and piston are used to apply an input force that is perpendicular to the cracking plane. Thus, no mechanism for converting force direction is required. On the other hand, the system of 4,754,906 is expensive to manufacture and maintain, and it is not well suited for production use on a continuous basis.
To assure that the connecting rod cracks exclusively within the desired plane, it is vital that there be no relative rotation between the main rod section and the cap during the cracking process. The prior machines and processes, such as are disclosed in the previously mentioned patents, often employ rather elaborate clamps to hold the main rod section and the cap rotationally immovable relative to each other. Such clamping adds to the expense of the cracking machinery and is further undesirable from a maintenance standpoint.
Another common characteristic of the prior connecting rod cracking devices is that they are limited to a large extent to use with connecting rods made of relatively brittle material or manufactured by powder metallurgy techniques. Attempts to crack relatively ductile materials have heretofore been unsuccessful from a commercial standpoint. U.S. Pat. No. 4,768,694 shows a connecting rod fracturing machine that includes cryogenic cooling stations for embrittling connecting rods prior to cracking them. Such cooling equipment and the related cooling processes are undesirably expensive and complicated.
A further drawback of the prior connecting rod cracking machinery is that they support the cracking dies, which extend through the crankshaft bore of the connecting rod, on only one end thereof. That is, the dies are supported in cantilever fashion on one side of the connecting rod. Consequently, the dies bend when the cracking forces are applied to them. Such bending results in non uniform stresses being produced in the webs along the cracking plane. Faulty cracks are a likely consequence.
European patent 0 396 797 shows a rod cracking system in which an impulsive force is applied to a connecting rod in a plane perpendicular to the longitudinal axis of the rod crankshaft bore. The connecting rod is placed over cantilever mounted fixed and movable dies. Separate clamps are installed between the free ends of the two dies and respective supports in an attempt to increase the rigidity of the dies. The separate clamps are cumbersome to handle and are impractical for commercial applications. A further disadvantage of the apparatus of European patent 0 396 797 and of several of the previously mentioned U.S. patents is that one or both of the dies are rigidly fixed to mechanisms that slide relative to the machine frame. During the cracking process, reaction forces are created between the sliding components, and those forces necessarily introduce friction into the system. Increased energy to overcome friction must therefore be applied in order to crack the rods.
Thus, a need exists for improved machines and processes for cracking connecting rods.